1. Field of the Invention
The present invention relates to a simple taking lens suitable for a low-priced camera, and more particularly to a taking lens that is constituted from two lens groups two lens elements.
2. Description of the Prior Art
In these days, a handy and simple camera such as lens-fitted photo film unit is produced and widely used. It is ready to take photographs at anytime, and, after taking exposures, it is just to be handed in a processing laboratory without taking out an exposed film. There are many types of the lens-fitted photo film units to fulfill users"" purposes and needs. It is sold at low-price, so its manufacturing costs should be as low as possible. Therefore, the lens-fitted photo film unit is of a simple construction in which a unit body pre-loaded with an unexposed photo film incorporates a simple photographic mechanism including a taking lens and a shutter mechanism.
The taking lens used for the lens-fitted photo film unit usually comprises one or two lenses, and has a fixed focus. The taking lens is secured in a lens holder in which a stop with a constant aperture is formed, and its focal length is predetermined to focus at any photographic scenes of which distances are different. In a lens-fitted photo film unit for a 35 mm size photo film, it is designed that a half angle of view is to be at 34xc2x0, a focal length of a taking lens is to be 32 mm and f-number thereof is to be about 9.5. In another lens-fitted photo film unit for a 24 mm size photo film, it is designed that a half angle of view is to be at 35xc2x0, a focal length of a taking lens is to be 24 mm and f-number thereof is to be about 9.5. Recently, in view of compensating aberration, it is mainly used a taking lens of two lens groups two lens elements type that comprises two lens groups in which each group has a single element, and is disclosed in JPA Nos. 9-329741, 7-333494, 5-281465, and 1-307712.
It is well known that the two lens groups two lens elements type comprises a first meniscus lens at an object side and a second lens with positive power at an image side, and each of the first and second lenses has an aspherical surface. The object side of the first lens has an aspherical surface, so it is difficult to keep a good image quality of an image formed outside of an optical axis while compensating aberration. Therefore, with the taking lens, it tends to enlarge the spherical aberration. If the taking lens is improved to make an illuminance of the image field brighter, an image formed on the center of the image surface is defected. To solve this problem, there is proposed with a taking lens comprising a first lens with an aspherical surface formed in its image side surface, instead of its object side surface.
In the suggested taking lens, a stop is disposed between the first and second lenses, whereas a radius of clear aperture on the image side surface of the second lens is too long comparing to the height of an axial bundle on the surface. Therefore, when using the taking lens with a shutter mechanism, so called a kicker shutter mechanism comprising a shutter blade that is kicked by a force of a coiled spring to open and close a shutter opening, and placing the shutter blade on the image side of the taking lens, the shutter opening becomes extremely large. Then, the unevenness of the exposure is occurred because that each swing interval of the shutter blade is different in response to a position of an image plane. If the shutter blade is disposed between the first and second lenses, called as a between lens shutter, then the shutter opening becomes smaller, and the unevenness of the exposure is reduced accordingly. However, the between lens shutter is complicated and increases a number of assembling steps, so suitability for assembling of the film unit is worsened.
In view of the foregoing problem, an object of the present invention is to provide a taking lens that a lens speed is fast to make an illuminance on an image surface brighter while keeping optical performance and suitability for assembling of a camera.
To achieve the above object, a taking lens of the present invention comprises of a first meniscus lens with its convex surface facing an object side, and a second lens having a positive power, arranged in this order from the object side. An image side surface of the first lens and either surface of the second lens are aspherical. The taking lens of the present invention satisfies following conditions:
0.25xe2x89xa6F/F1xe2x89xa60.39xe2x80x83xe2x80x83(1)
xe2x88x922.5xe2x89xa6F/R3xc3x97fnoxe2x89xa61.5xe2x80x83xe2x80x83(2)
wherein F represents a focal length of the taking lens, F1 represents a focal length of the first lens, R3 represents a radius of curvature of the object side surface of the second lens, and fno represents f-number of the taking lens at open aperture.
In preferred embodiments of the present invention, a stop is disposed behind the second lens, whereas, between the first and second lenses, is provided a flare stop that restricts light paths of off-axial light rays that focused on out of an optical axis at an image plane. The flare stop satisfies a following condition:
1.0xe2x89xa6E1/H1xe2x89xa61.3xe2x80x83xe2x80x83(3)
wherein E1 represents a radius of a clear aperture of the flare stop, and H1 represents a height of an axial bundle from the optical axis on the surface of the flare stop.
Moreover, in the preferred embodiments of the present invention, the stop is placed between the first and second lenses, and a radius of a clear aperture on the image side surface of the second lens is enlarged to assuredly gain much brightness of the margin of the image field. The second lens satisfies a following condition:
1.0xe2x89xa6E2/H2xe2x89xa61.3xe2x80x83xe2x80x83(4)
wherein E2 represent the radius of the clear aperture on the image side surface of the second lens, and H2 represents a height of the axial bundle from the optical axis on the image side surface of the second lens.
In case of disposing the stop on the image side surface of the second lens, the longer an inner diameter of the flare stop provided between the first and second lenses, the more amount of light on the margin of the image field can be kept. However, above the upper limit of the condition (3), there is too much amount of light rays passing through lower side the optical axis at a position of the stop among the light rays directing toward a position of 25 percent of the height from the optical axis at the image surface, and then the flare in a picture frame increases.
Similarly, above the upper limit of the condition (4), the flare in the picture frame increases, so the entire photographic image is extremely defected. Moreover, the light rays traveling from the second lens to the image surface are widely spread so that the shutter opening should be large enough to allow the passing light rays when disposing the kicker shutter mechanism on the image side surface of the second lens. Then, the unevenness of the exposure is likely to occur.
Moreover, because of the above construction, the illuminance of the image surface becomes brighter. Therefore, it is possible to control exposure by inserting a smaller stop between the first and second lenses aside from a stop aperture even when using the taking lens with a shutter mechanism having a constant shutter speed.
According to the present invention, the first lens has not an aspherical surface on its object side, so it is possible to restrain enlarging a spherical aberration and to obtain a good image on entire image surface. In addition, by arranging the focal length of the first lens and a relationship between the radius of curvature of the object side surface of the second lens and the f-number of the taking lens of the film unit, it is possible to provide the taking lens having a brighter illuminance without unbalancing aberrations.
Furthermore, between the image side surface of the first lens and the image side surface of the second lens, a height of the off-axial rays becomes lower than the height of light rays from the optical axis, so the light rays through the lenses are not affected as far as the stop is positioned therebetween. Therefore, in the taking lens of the present invention, even if the stop is positioned either on the image side surface of the second lens or between the first and second lenses, no changes happen to the lens quality and the illuminance of the image field. It is possible to design the lenses without considering their positional relationship with the stop. Moreover, the image side surface of the second lens is limited to have the minimum clear aperture, so even when the kicker shutter mechanism is disposed thereupon, the shutter opening does not become large to the degree in which the unevenness of the exposure occur. Thus, it is possible to obtain a good image.